1. Field of the Invention
The present invention relates to a system for bi-directional communication between a station and a plurality of terminals, and more specifically to a scramble communication system which is capable of improving the secrecy of communication.
2. Description of the Prior Art
There have been proposed a variety of scramble communication systems which are designed to improve the secrecy of communication.
As a first example of such a scramble communication system, a voice conference system is disclosed in Japanese Patent Application Laid-open No. SHO 57-103464. The voice conference system is comprised of a loop transmission line to which a plurality of subscriber-terminals are connected. Each subscriber terminal is provided with a scrambler which is capable of selecting one of several scramble generator polynomials. By using the same scramble generator polynomial, secret communications can be made in a certain group of subscriber terminals.
A second example is a communication system employing encrypted data, which is disclosed in Japanese Patent Application Laid-open No. SHO 59-134939. This communication system is comprised of a plurality of digital terminals which are connected to each other through a digital subscriber circuit. According to this system, an encryption pattern used in a call is determined by the calling terminal and the called terminal transmitting an encryption pattern to each other.
A third example is a one-way TDM (Time Division Multiplexing) system described in Japanese Patent Application Laid-open No. SHO 63-10833. According to this system, in a transmission side, different channels (not all channels) are scrambled based on different generator polynomials, and in a receiving side, the respective channels are descrambled based on the same generator polynomials.
A bi-directional communication system having a configuration different from the three systems described above is illustrated in FIG. 1. Referring to the figure, optical signals transmitted from a plurality of terminals 1 are combined through a star coupler 2, and the combined signal is received by a station 3. In contrast, an optical signal transmitted from the station 3 is distributed through a star coupler 4 to all the terminals 1. Each terminal 1 is provided with an electric/optical signal converter 5 for converting a transmission signal to an optical signal, an optical/electric signal converter 6 for converting a received optical signal to an electric signal, and a descramble circuit 7 for releasing scrambling. The station 3 is comprised of an optical/electric signal converter 8 for converting a received optical signal to an electric signal, a scramble circuit 9 for scrambling a transmission signal, and an electric/optical signal converter 10 for converting the scrambled signal to an optical signal.
For transmitting a signal from the station 3 to the terminals 1, the signal is first scrambled by the scramble circuit 9 using a predetermined scramble generator polynomial. The scrambled signal is converted to an optical signal which is output to the star coupler 4. In this manner, the optical signal is distributed to the terminals 1 in which the received optical signal is descrambled using the same scramble generator polynomial as in the station 3 for reproduction of received data. Such a scramble system enables information distribution to all terminals which are provided in advance with the same scramble generator polynomial.
In the bi-directional scramble communication system, however, an optical signal is distributed from the station 3 to all terminals 1 though the star coupler 4, so that selective communications between the station 3 and one or more of the terminals 1 are impossible.
It is conceivable to apply a scrambler capable of selecting one of several scramble generator polynomials disclosed in the above first example to the station 3 of the bi-directional scramble communication system. However, in a kind of system having many subscribers such as a broadcasting system, a large number of scramble generator polynomials must be registered and stored in the station 3. For the same reason, the TDM scramble communication system as disclosed in the third example is not practical in such a system having a large number of subscriber terminals.
It is also conceivable to apply an encryption communication system disclosed in the above second example to the bi-directional scramble communication system. However, since the encryption pattern used in the communication must be determined by every terminal 1 and the station 3, a large number of subscriber terminals cannot be handled.